Wafer-level package with enhanced performance

ABSTRACT

The present disclosure relates to a wafer-level package that includes a first thinned die having a first device layer, a multilayer redistribution structure, a first mold compound, and a second mold compound. The multilayer redistribution structure includes redistribution interconnects that connect the first device layer to package contacts on a bottom surface of the multilayer redistribution structure. Herein, the connections between the redistribution interconnects and the first device layer are solder-free. The first mold compound resides over the multilayer redistribution structure and around the first thinned die, and extends beyond a top surface of the first thinned die to define an opening within the first mold compound and over the first thinned die. The second mold compound fills the opening and is in contact with the top surface of the first thinned die.

RELATED APPLICATIONS

This application claims the benefit of provisional patent application serial number 62/339,322, filed May 20, 2016, the disclosure of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a wafer-level package and a process for making the same, and more particularly to a wafer-level package with enhanced thermal and electrical performance, and a packaging process to enhance thermal and electrical performance of a wafer-level package.

BACKGROUND

The wide utilization of cellular and wireless devices drives the rapid development of radio frequency (RF) technologies. The substrates on which RF devices are fabricated play an important role in achieving high level performance in the RF technologies. Fabrications of the RF devices on conventional silicon substrates may benefit from low cost of silicon materials, a large scale capacity of wafer production, well-established semiconductor design tools, and well-established semiconductor manufacturing techniques.

Despite the benefits of using conventional silicon substrates for the RF device fabrications, it is well known in the industry that the conventional silicon substrates may have two undesirable properties for the RF devices: harmonic distortion and low resistivity values. The harmonic distortion is a critical impediment to achieve high level linearity in the RF devices built over silicon substrates. In addition, the low resistivity encountered in the silicon substrates may degrade quality factors (Q) at high frequencies of microelectromechanical systems (MEMS) or other passive components.

In addition, high speed and high performance transistors are more densely integrated in RF devices. Consequently, the amount of heat generated by the RF devices will increase significantly due to the large number of transistors integrated in the RF devices, the large amount of power passing through the transistors, and the high operation speed of the transistors. Accordingly, it is desirable to package the RF devices in a configuration for better heat dissipation.

Wafer-level fan-out (WLFO) packaging technology and embedded wafer-level ball grid array (EWLB) technology currently attract substantial attention in portable RF applications. WLFO and EWLB technologies are designed to provide high density input/output ports (I/O) without increasing the size of a package. This capability allows for densely packaging the RF devices within a single wafer.

To accommodate the increased heat generation of the RF devices, to reduce deleterious harmonic distortion of the RF devices, and to utilize advantages of WLFO/EWLB packaging technologies, it is therefore an object of the present disclosure to provide an improved package design with enhanced thermal and electrical performance. Further, there is also a need to enhance the performance of the RF devices without increasing the package size.

SUMMARY

The present disclosure relates to a wafer-level package with enhanced thermal and electrical performance, and a packaging process to enhance thermal and electrical performance of a wafer-level package. The disclosed wafer-level package includes a first thinned die, a multilayer redistribution structure, a first mold compound, and a second mold compound. The first thinned die includes a first device layer and a first dielectric layer over the first device layer. Herein, the first device layer includes a number of first die contacts at a bottom surface of the first device layer. The multilayer redistribution structure includes a number of package contacts on a bottom surface of the multilayer redistribution structure and redistribution interconnects that connect the first die contacts to certain ones of the package contacts. The multilayer redistribution structure is free of glass fiber and connections between the redistribution interconnects and the first die contacts are solder-free. In addition, the first mold compound resides over the multilayer redistribution structure and around the first thinned die, and extends beyond a top surface of the first thinned die to define an opening within the first mold compound and over the first thinned die. The top surface of the first thinned die is exposed at a bottom of the opening. The second mold compound fills the opening and is in contact with the top surface of the first thinned die.

In one embodiment of the wafer-level package, the first thinned die provides a microelectromechanical systems (MEMS) component.

In one embodiment of the wafer-level package, the first thinned die is formed from a silicon-on-insulator (SOI) structure. The first device layer of the first thinned die is formed from a silicon epitaxy layer of the SOI structure, and the first dielectric layer of the first thinned die is a buried oxide layer of the SOI structure.

According to another embodiment, the wafer-level package further includes a second intact die residing over the multilayer redistribution structure.

Herein, the second intact die has a second device layer and an intact silicon substrate over the second device layer, and the first mold compound encapsulates the second intact die.

According to another embodiment, the wafer-level package further includes a third thinned die residing over the multilayer redistribution structure.

Herein, the third thinned die has a third device layer and a second dielectric layer over the third device layer. The first mold compound extends beyond a top surface of the third thinned die to define a second opening within the first mold compound and over the third thinned die. The top surface of the third thinned die is exposed at a bottom of the second opening. The second mold compound fills the second opening and in contact with the top surface of the third thinned die.

In one embodiment of the wafer-level package, the first thinned die provides a MEMS component, the second intact die provides a complementary metal-oxide-semiconductor (CMOS) controller that controls the MEMS component, and the third thinned die is formed from a SOI structure. Herein, the third device layer of the third thinned die is formed from a silicon epitaxy layer of the SOI structure, and the second dielectric layer of the third thinned die is a buried oxide layer of the SOI structure.

In one embodiment of the wafer-level package, the second mold compound has a thermal conductivity greater than 2 W/m·K.

In one embodiment of the wafer-level package, the second mold compound has a thermal conductivity greater than 10 W/m·K.

In one embodiment of the wafer-level package, the second mold compound has an electrical resistivity greater that 1 E6 Ohm-cm.

In one embodiment of the wafer-level package, the first mold compound and the second mold compound are formed from different materials.

In one embodiment of the wafer-level package, the first mold compound is formed from a same material as the second mold compound.

In one embodiment of the wafer-level package, the first mold compound is formed from a different material from the second mold compound.

In one embodiment of the wafer-level package, the bottom surface of the first device layer is in contact with the multilayer redistribution structure, and the first die contacts are directly connected to the redistribution interconnects.

In one embodiment of the wafer-level package, the first thinned die further includes a number of first pillars. Each of the first pillars extends from the first device layer to the multilayer redistribution structure and couples one of the first die contacts to a corresponding redistribution interconnect.

According to another embodiment, the wafer-level package further includes an underfilling layer that resides between the multilayer redistribution structure and the first mold compound, and underfills the first thinned die to encapsulate the first pillars.

In one embodiment of the wafer-level package, the underfilling layer is formed from a same material as the first mold compound.

In one embodiment of the wafer-level package, the top surface of the first thinned die exposed at the bottom of the opening is a top surface of the first dielectric layer.

In one embodiment of the wafer-level package, the first dielectric layer is formed from one of a group consisting of silicon oxide, silicon nitride, and aluminum nitride.

According to an exemplary process, a mold wafer having a first die and a first mold compound is provided. Herein, the first die includes a first device layer, a first dielectric layer over the first device layer, and a first silicon substrate over the first dielectric layer. The first device layer includes a number of first die contacts at a bottom surface of the first device layer. The first mold compound encapsulates sides of the first die such that the bottom surface of the first device layer and a top surface of the first silicon substrate are exposed. Next, a multilayer redistribution structure is formed underneath the mold wafer. The multilayer redistribution structure includes a number of package contacts on a bottom surface of the multilayer redistribution structure and redistribution interconnects that connect the first die contacts to certain ones of the package contacts. The multilayer redistribution structure is free of glass fiber and connections between the redistribution interconnects and the first die contacts are solder-free. The first silicon substrate of the first die is then removed substantially to provide a first thinned die and form a first opening within the first mold compound and over the first thinned die. A top surface of the first thinned die is exposed at a bottom of the opening. Lastly, a second mold compound is applied to substantially fill the opening and directly contact the top surface of the first thinned die.

Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

FIG. 1 shows an exemplary wafer-level package with enhanced thermal and electrical performance according to one embodiment of the present disclosure.

FIG. 2 shows an alternative wafer-level package with enhanced thermal and electrical performance according to one embodiment of the present disclosure.

FIGS. 3-14 provide exemplary steps that illustrate a process to fabricate the exemplary wafer-level package shown in FIG. 1.

It will be understood that for clear illustrations, FIGS. 1-14 may not be drawn to scale.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to a wafer-level package with enhanced thermal and electrical performance, and a packaging process to enhance thermal and electrical performance of a wafer-level package. FIG. 1 shows an exemplary wafer-level package 10 according to one embodiment of the present disclosure. For the purpose of this illustration, the exemplary wafer-level package 10 includes a thinned silicon-on-insulator (SOI) die 12, a thinned microelectromechanical systems (MEMS) die 14, a complementary metal-oxide-semiconductor (CMOS) controller die 16, a multilayer redistribution structure 18, a first mold compound 20, and a second mold compound 22. In different applications, the wafer-level package 10 may include fewer or more thinned MEMS/SOl dies and may include other dies, such as: thinned integrated passive device dies (not shown). For instance, in some applications, the wafer-level package 10 may only include thinned MEMS dies and CMOS controller dies; and in some applications, the wafer-level package 10 may only include thinned SOI dies.

In detail, the thinned SOI die 12 includes a first device layer 24 and a first dielectric layer 26 over a top surface of the first device layer 24. The first device layer 24 includes a number of first die contacts 28 at a bottom surface of the first device layer 24. Herein, the thinned SOI die 12 is formed from an SOI structure, which refers to a structure including a silicon substrate, a silicon epitaxy layer, and a buried oxide layer sandwiched between the silicon substrate and the silicon epitaxy layer. The first device layer 24 of the thinned SOI die 12 is formed by integrating electronic components (not shown) in or on the silicon epitaxy layer of the SOI structure. The first dielectric layer 26 of the thinned SOI die 12 is the buried oxide (BOX) layer of the SOI structure. In addition, the silicon substrate of the SOI structure is removed substantially to complete the thinned SOI die 12 (more details in the following discussion). The first device layer 24 has a thickness between 0.1 μm and 50 μm, and the first dielectric layer 26 has a thickness between 10 nm and 1000 nm.

The thinned MEMS die 14 includes a second device layer 30 and a second dielectric layer 32 over a top surface of the second device layer 30. The second device layer 30 includes a MEMS component 34 that is typically a switch, a number of second die contacts 36 at a bottom surface of the second device layer 30, and MEMS via-structures 38 extending from the MEMS component 34 to the second die contacts 36. Herein, the second device layer 30 may include a cavity, within which the MEMS component 34 is free to actuate. The second device layer 30 has a thickness between 0.5 μm and 100 μm, and may be formed from a combination of dielectric and metal layers (such as oxide, nitride, aluminum, titanium, copper, or the like). The second dielectric layer 32 has a thickness between 10 nm and 5000 nm, and may be formed from silicon oxide, silicon nitride, or aluminum nitride.

Notice that the thinned SOI die 12 and the thinned MEMS die 14 are both thinned dies, which have a device layer, a dielectric layer over the device layer, and essentially no silicon substrate over the dielectric layer. Herein, essentially no silicon substrate over the dielectric layer refers to at most 2 μm silicon substrate over the dielectric layer. In desired cases, each thinned die does not include any silicon substrate over the dielectric layer such that a top surface of each thinned die is a top surface of the dielectric layer. For other cases, the top surface of one thinned die may be a top surface of the thin silicon substrate.

The CMOS controller die 16 includes a third device layer 40 and a silicon substrate 42 over the third device layer 40. The third device layer 40 includes a CMOS controller 44 that controls the MEMS component 34 within the thinned MEMS die 14, a number of third die contacts 46 at a bottom surface of the third device layer 40, and controller via-structures 48 extending from the CMOS controller 44 to the third die contacts 46. The third device layer 40 has a thickness between 0.5 μm and 20 μm, and may be formed from a combination of dielectric and metal layers (such as oxide, nitride, aluminum, titanium, copper, or the like). The CMOS controller die 16 is an intact die, which includes an intact silicon substrate with a thickness between 50 μm and 250 μmor between 50 μm and 750 μm.

Herein, the multilayer redistribution structure 18 includes a first dielectric pattern 50 at the top, a number of redistribution interconnects 52, a second dielectric pattern 54, and a number of package contacts 56. In one embodiment, the thinned SOI die 12, the thinned MEMS die 14, and the CMOS controller die 16 reside directly over the multilayer redistribution structure 18. As such, the first device layer 24 of the thinned SOI die 12, the second device layer 30 of the thinned MEMS die 14, and the third device layer 40 of the CMOS controller die 16 are in contact with the first dielectric pattern 50. In addition, the first die contacts 28 at the bottom surface of the first device layer 24, the second die contacts 36 at the bottom surface of the second device layer 30, and the third die contacts 46 at the bottom surface of the third device layer 40 are exposed through the first dielectric pattern 50. The redistribution interconnects 52 are electrically coupled to the first, second, and third die contacts 28, 36, and 46 through the first dielectric pattern 50 and extend underneath the first dielectric pattern 50. The connections between the redistribution interconnects 52 and the first, second, and third die contacts 28, 36, and 46 are solder-free.

The second dielectric pattern 54 is formed underneath the first dielectric pattern 50 to partially encapsulate each redistribution interconnect 52.

As such, a portion of each redistribution interconnect 52 is exposed through the second dielectric pattern 54. In different applications, there may be extra redistribution interconnects (not shown) electronically coupled to the redistribution interconnects 52 through the second dielectric pattern 54, and an extra dielectric pattern (not shown) formed underneath the second dielectric pattern 54 to partially encapsulate the extra redistribution interconnects.

In this embodiment, each package contact 56 is electronically coupled to a corresponding redistribution interconnect 52 through the second dielectric pattern 54. Consequently, the redistribution interconnects 52 connect certain ones of the first, second, and third die contacts 28, 36, and 46 to certain ones of the package contacts 56 on a bottom surface of the multilayer redistribution structure 18.

The multilayer redistribution structure 18 may be free of glass fiber or glass-free. Herein, the glass fiber refers to individual glass strands twisted to become a larger grouping. These glass strands may be then woven into a fabric.

The first dielectric pattern 50 and the second dielectric pattern 54 may be formed of benzocyclobutene (BCB) or polyimide. The redistribution interconnects 52 may be formed of copper or other suitable metals. The package contacts 56 may be bump contacts formed of solder alloys, such as tin or tin alloys, or may be land grid arrays (LGA) contacts. A combination of the first dielectric pattern 50, the redistribution interconnects 52, and the second dielectric pattern 54 has a thickness between 2 μm and 300 μm.

The first mold compound 20 resides over a top surface of the multilayer redistribution structure 18, resides around the thinned SOI die 12 and the thinned MEMS die 14, and encapsulates the CMOS controller die 16.

Further, the first mold compound 20 extends beyond a top surface of the thinned SOI die 12 to define a first opening 58 within the first mold compound 20 and over the thinned SOI die12, and extends beyond a top surface of the thinned MEMS die 14 to define a second opening 60 within the first mold compound 20 and over the thinned MEMS die14. Herein, the top surface of the thinned SOI die 12 is exposed at a bottom of the first opening 58, and the top surface of the thinned MEMS die 14 is exposed at a bottom of the second opening 60.

The second mold compound 22 substantially fills the first and second openings 58 and 60, and is in contact with the top surface of the thinned SOI die 12 and the top surface of the thinned MEMS die 14. The second mold compound 22 has a thermal conductivity greater than 2 W/m·K or greater than 10 W/m·K, and has an electrical resistivity greater than 1 E6 Ohm-cm. In general, the higher the thermal conductivity of the second mold compound 22, the better the thermal performance of the thinned SOI die 12 and the thinned MEMS die 14. Further, the high electrical resistivity of the second mold compound 22 may improve the quality factor (Q) at high frequencies of the MEMS component 34 of the thinned MEMS die 14.

The second mold compound 22 may be formed of thermoplastics or thermoset materials, such as PPS (poly phenyl sulfide), overmold epoxies doped with boron nitride or alumina thermal additives, or the like. The second mold compound 22 may be formed of the same or different material as the first mold compound 20. However, unlike the second mold compound 22, the first mold compound 20 does not have thermal conductivity or electrical resistivity requirements. In some applications, both the first mold compound 20 and the second mold compound 22 have a thermal conductivity greater than 2 W/m·K. In some applications, the first mold compound 20 has a thermal conductivity less than 2 W/m·K and the second mold compound 22 has a thermal conductivity greater than 2 W/m·K. In some applications, the first mold compound 20 has a thermal conductivity greater than 2 W/m·K and the second mold compound 22 has a thermal conductivity greater than 10 W/m·K. Herein, a portion of the second mold compound 22 may reside over a top surface of the first mold compound 20. Notice that the second mold compound 22 is separate from the CMOS controller die 16 by the first mold compound 20. A top surface of the CMOS controller die 16 is in contact with the first mold compound 20.

In another embodiment, the thinned SOI die 12, the thinned MEMS die 14, and the CMOS controller die 16 are connected to the multilayer redistribution structure 18 through a number of pillars as illustrated in FIG. 2. The thinned SOI die 12 further includes first pillars 62 that extend from the first device layer 24 to the multilayer redistribution structure 18 and couple the first die contacts 28 to the corresponding redistribution interconnects 52. The thinned MEMS die 14 further includes second pillars 64 that extend from the second device layer 30 to the multilayer redistribution structure 18 and couple the second die contacts 36 to the corresponding redistribution interconnects 52. The CMOS controller die 16 further includes third pillars 66 that extend from the third device layer 40 to the multilayer redistribution structure 18 and couple the third die contacts 46 to the corresponding redistribution interconnects 52. Since the first, second and third pillars 62, 64, and 66 are formed from electrically conductive materials, such as copper, the first, second, and third die contacts 28, 36, and 46 are still electrically coupled to corresponding package contacts 56.

In some applications, the wafer-level package 10 may further include an underfilling layer 68 to encapsulate the first, second, and third pillars 62, 64, and 66. The underfilling layer 68 resides between the top surface of the multilayer redistribution structure 18 and the first mold compound 20, and underfills the thinned SOI die 12, the thinned MEMS die 14, and the CMOS controller die 16. The underfilling layer 68 may be formed from the same or different material as the first mold compound 20. In some applications, the first mold compound 20 and the underfilling layer 68 may have a thermal conductivity greater than 2 W/m·K.

FIGS. 3-14 provide exemplary steps to fabricate the exemplary wafer-level package 10 shown in FIG. 1. Although the exemplary steps are illustrated in a series, the exemplary steps are not necessarily order dependent.

Some steps may be done in a different order than that presented. Further, processes within the scope of this disclosure may include fewer or more steps than those illustrated in FIGS. 3-14.

Initially, an adhesive layer 70 is applied on a top surface of a carrier 72 as illustrated in FIG. 3. Then, an SOI die 12D, a MEMS die 14D, and the

CMOS controller die 16 are attached to the adhesive layer 70 as illustrated in FIG. 4. In different applications, there might be fewer or more dies attached to the adhesive layer 70. For instance, in some applications, there may be only the SOI die 12D attached to the adhesive layer 70; and in some applications, there may be only the MEMS die 14D and the CMOS controller die 16 attached to the adhesive layer 70.

For the purpose of this illustration, the SOI die 12D includes the first device layer 24, the first dielectric layer 26 over the top surface of the first device layer 24, and a first silicon substrate 74 over the first dielectric layer 26. As such, the bottom surface of the first device layer 24 is a bottom surface of the SOI die 12D, and the backside of the first silicon substrate 74 is a top surface of the SOI die 12D. Herein, the SOI die 12D is formed from an SOI structure, which refers to a structure including a silicon substrate, a silicon epitaxy layer, and a buried oxide layer sandwiched between the silicon substrate and the silicon epitaxy layer. The first device layer 24 of the SOI die 12D is formed by integrating electronic components (not shown) in or on the silicon epitaxy layer of the SOI structure. The first dielectric layer 26 of the SOI die 12D is the buried oxide layer of the SOI structure. The first silicon substrate 74 of the SOI die 12D is the silicon substrate of the SOI structure. The SOI die 12D has a thickness between 25 μm and 250 μm or between 25 μm and 750 μm, and the first silicon substrate 74 has a thickness between 25 μm and 250 μm or between 25 μm and 750 μm, respectively.

The MEMS die 14D includes the second device layer 30, the second dielectric layer 32 over the top surface of the second device layer 30, and a second silicon substrate 76 over the second dielectric layer 32. As such, the bottom surface of the second device layer 30 is a bottom surface of the MEMS die 14D, and the backside of the second silicon substrate is a top surface of the MEMS die 14D. The MEMS die 14D has a thickness between 50 μm and 300 μm or between 50 μm and 800 μm, and the second silicon substrate 76 has a thickness between 50 μm and 250 μm or between 50 μm and 750 μm, respectively. In this embodiment, both the SOI die 12D and the MEMS die 14D are taller than the CMOS controller die 16. If there are only the MEMS die 14D and the CMOS controller die 16 attached to the adhesive layer 70, the CMOS controller die 16 is just shorter than the MEMS die 14D.

Next, the first mold compound 20 is applied over the adhesive layer 70 to encapsulate the SOI die 12D, the MEMS die 14D, and the CMOS controller die 16 as illustrated in FIG. 5. The first mold compound 20 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, or screen print encapsulation. The first mold compound 20 may be an organic epoxy resin system or the like, which can be used as an etchant barrier to protect the SOI die 12D, the MEMS die 14D, and the CMOS controller die 16 against etching chemistries such as potassium hydroxide (KOH), sodium hydroxide (NaOH), and acetylcholine (ACH). A curing process (not shown) is then used to harden the first mold compound 20. The curing temperature is between 100° C. and 320° C. depending on which material is used as the first mold compound 20. The adhesive layer 70 and the carrier 72 are then removed to expose the bottom surface of the first device layer 24, the bottom surface of the second device layer 30, and the bottom surface of the third device layer 40 as shown in FIG. 6. Removal of the adhesive layer 70 and the carrier 72 may be provided by heating the adhesive layer 70.

The first mold compound 20 is then thinned down to expose the first silicon substrate 74 of the SOI die 12D and the second silicon substrate 76 of the

MEMS die 14D as shown in FIG. 7. The thinning procedure may be done with a mechanical grinding process. Since the CMOS controller die 16 has a lower height than both the MEMS die 14D and the SOI die 12D, the silicon substrate 42 of the CMOS controller die 16 is not exposed and still encapsulated by the first mold compound 20.

With reference to FIGS. 8 through 11, the multilayer redistribution structure 18 is formed according to one embodiment of the present disclosure. The first dielectric pattern 50 is firstly formed underneath the SOI die 12D, the MEMS die 14D, and the CMOS controller die 16, as illustrated in FIG. 8. As such, the first, second, and third die contacts 28, 36, and 46 are exposed through the first dielectric pattern 50. Next, the redistribution interconnects 52 are formed as illustrated in FIG. 9. The redistribution interconnects 52 are electrically coupled to the first, second, and third die contacts 28, 36, and 46 through the first dielectric pattern 50 and extend underneath the first dielectric pattern 50. Notice that the connections between the redistribution interconnects 52 and the first, second, and third die contacts 28, 36, and 46 are solder-free.

The second dielectric pattern 54 is formed underneath the first dielectric pattern 50 to partially encapsulate each redistribution interconnect 52 as illustrated in FIG. 10. As such, a portion of each redistribution interconnect 52 is exposed through the second dielectric pattern 54. Lastly, the package contacts 56 are applied to exposed portions of the redistribution interconnects 52 as illustrated in FIG. 11. The package contacts 56 may be applied by a standard bumping procedure or a land grid arrays process. Herein, the redistribution interconnects 52 connect certain ones of the first, second, and third die contacts 28, 36, and 46 to certain ones of the package contacts 56 on a bottom surface of the multilayer redistribution structure 18.

After the multilayer redistribution structure 18 is formed, the first silicon substrate 74 and the second silicon substrate 76 are removed substantially as illustrated in FIG. 12. The removal of the first silicon substrate 74 from the SOI die 12D provides the thinned SOI die 12 and forms the first opening 58 within the first mold compound 20 and over the thinned SOI die 12. The removal of the second silicon substrate 76 from the MEMS die 14D provides the thinned MEMS die 14 and forms the second opening 60 within the first mold compound 20 and over the thinned MEMS die 14. Herein, removing substantially a silicon substrate refers to removing at least 95% of the entire silicon substrate and remaining at most 2 pm silicon substrate. In desired cases, the first and second silicon substrates 74 and 76 are removed completely, such that the first dielectric layer 26 of the thinned SOI die 12 is exposed at the bottom of the first opening 58 and the second dielectric layer 32 of the thinned MEMS die 14 is exposed at the bottom of the second opening 60. Removing substantially the first and second silicon substrates 74 and 76 may be provided by an etching process with a wet/dry etchant chemistry, which may be TMAH, KOH, ACH, NaOH, or the like.

The second mold compound 22 is then applied to substantially fill the first and second openings 58 and 60, as illustrated in FIG. 13. The second mold compound 22 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, and screen print encapsulation. The second mold compound 22 directly resides over the top surface of the thinned SOI die 12 and the top surface of the thinned MEMS die 14. If there is no first silicon substrate 74 left in the first opening 58 and no second silicon substrate 76 left in the second opening 60, the second mold compound 22 directly resides over the first dielectric layer 26 and the second dielectric layer 32. In some cases, the second mold compound 22 may further reside over the first mold compound 20. A curing process (not shown) is followed to harden the second mold compound 22. The curing temperature is between 100° C. and 320° C. depending on which material is used as the second mold compound 22.

A top surface of the second mold compound 22 is then planarized to form the wafer-level package 10 as illustrated in FIG. 14. A mechanical grinding process may be used for planarization. Lastly, the wafer-level semiconductor package 10 may be marked, diced, and singulated into individual components (now shown).

Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. An apparatus comprising: a first thinned die comprising a first device layer and a first dielectric layer over the first device layer, wherein the first device layer comprises a plurality of first die contacts at a bottom surface of the first device layer; a multilayer redistribution structure comprising a plurality of package contacts on a bottom surface of the multilayer redistribution structure and redistribution interconnects that connect the plurality of first die contacts to certain ones of the plurality of package contacts, wherein the multilayer redistribution structure is free of glass fiber and connections between the redistribution interconnects and the plurality of first die contacts are solder-free; a first mold compound residing over the multilayer redistribution structure and around the first thinned die, and extending beyond a top surface of the first thinned die to define an opening within the first mold compound and over the first thinned die, wherein the top surface of the first thinned die is exposed at a bottom of the opening; and a second mold compound filling the opening and in contact with the top surface of the first thinned die.
 2. The apparatus of claim 1, wherein the first thinned die provides a microelectromechanical systems (MEMS) component.
 3. The apparatus of claim 1, wherein the first thinned die is formed from a silicon-on-insulator (SOI) structure, wherein the first device layer of the first thinned die is formed from a silicon epitaxy layer of the SOI structure, and the first dielectric layer of the first thinned die is a buried oxide layer of the SOI structure.
 4. The apparatus of claim 1 further comprising a second intact die residing over the multilayer redistribution structure, wherein: the second intact die has a second device layer and an intact silicon substrate over the second device layer; and the first mold compound encapsulates the second intact die.
 5. The apparatus of claim 4, wherein the first thinned die provides a MEMS component and the second intact die provides a complementary metal-oxide-semiconductor (CMOS) controller that controls the MEMS component.
 6. The apparatus of claim 4 further comprising a third thinned die residing over the multilayer redistribution structure, wherein: the third thinned die has a third device layer and a second dielectric layer over the third device layer; the first mold compound extends beyond a top surface of the third thinned die to define a second opening within the first mold compound and over the third thinned die, wherein the top surface of the third thinned die is exposed at a bottom of the second opening; and the second mold compound fills the second opening and in contact with the top surface of the third thinned die.
 7. The apparatus of claim 6, wherein the first thinned die provides a MEMS component, the second intact die provides a CMOS controller that controls the MEMS component, and the third thinned die is formed from a SOI structure, wherein the third device layer of the third thinned die is formed from a silicon epitaxy layer of the SOI structure, and the second dielectric layer of the third thinned die is a buried oxide layer of the SOI structure.
 8. The apparatus of claim 1, wherein the second mold compound has a thermal conductivity greater than 2 W/m·K.
 9. The apparatus of claim 1, wherein the second mold compound has a thermal conductivity greater than 10 W/m·K.
 10. The apparatus of claim 1, wherein the second mold compound has an electrical resistivity greater that 1 E6 Ohm-cm.
 11. The apparatus of claim 1, wherein the first mold compound is formed from a same material as the second mold compound.
 12. The apparatus of claim 11, wherein the first mold compound and the second mold compound have a thermal conductivity greater than 2 W/m·K.
 13. The apparatus of claim 1, wherein the first mold compound and the second mold compound are formed from different materials.
 14. The apparatus of claim 13, wherein the first mold compound has a thermal conductivity greater than 2 W/m·K and the second mold compound has a thermal conductivity greater than 10 W/m·K.
 15. The apparatus of claim 1, wherein the bottom surface of the first device layer is in contact with the multilayer redistribution structure, and the plurality of first die contacts are directly connected to the redistribution interconnects.
 16. The apparatus of claim 1, wherein the first thinned die further comprises a plurality of first pillars, wherein each of the plurality of first pillars extends from the first device layer to the multilayer redistribution structure and couples one of the plurality of first die contacts to a corresponding redistribution interconnect.
 17. The apparatus of claim 16 further comprising an underfilling layer that resides between the multilayer redistribution structure and the first mold compound, and underfills the first thinned die to encapsulate the plurality of first pillars.
 18. The apparatus of claim 17 wherein the underfilling layer is formed from a same material as the first mold compound.
 19. The apparatus of claim 18, wherein the first mold compound and the underfilling layer have a thermal conductivity greater than 2 W/m·K.
 20. The apparatus of claim 1, wherein the top surface of the first thinned die exposed at the bottom of the opening is a top surface of the first dielectric layer.
 21. The apparatus of claim 1, wherein the first dielectric layer is formed from one of a group consisting of silicon oxide, silicon nitride, and aluminum nitride.
 22. The apparatus of claim 1, wherein the plurality of package contacts are bump contacts or land grid arrays (LGA) contacts.
 23. The apparatus of claim 1, wherein the multilayer redistribution structure is glass-free.
 24. A method comprising: providing a mold wafer having a first die and a first mold compound, wherein: the first die comprises a first device layer, a first dielectric layer over the first device layer, and a first silicon substrate over the first dielectric layer, wherein the first device layer comprises a plurality of first die contacts at a bottom surface of the first device layer; and the first mold compound encapsulates sides of the first die, wherein the bottom surface of the first device layer and a top surface of the first silicon substrate are exposed; forming a multilayer redistribution structure underneath the mold wafer, wherein: the multilayer redistribution structure comprises a plurality of package contacts on a bottom surface of the multilayer redistribution structure and redistribution interconnects that connect the plurality of first die contacts to certain ones of the plurality of package contacts; the multilayer redistribution structure is free of glass fiber; and connections between the redistribution interconnects and the plurality of first die contacts are solder-free; removing substantially the first silicon substrate of the first die to provide a first thinned die and form a first opening within the first mold compound and over the first thinned die, wherein the first thinned die has a top surface exposed at a bottom of the opening; and applying a second mold compound to substantially fill the opening and directly contact the top surface of the first thinned die.
 25. The method of claim 24 wherein providing the mold wafer comprises: applying an adhesive layer over a carrier; attaching the first die to the adhesive layer, wherein the plurality of first die contacts at the bottom surface of the first device layer are covered by the adhesive layer; applying the first mold compound over the adhesive layer to encapsulate the first die; removing the carrier and the adhesive layer to expose the bottom surface of the first device layer; and thinning down the first mold compound to expose the top surface of the first silicon substrate.
 26. The method of claim 24, wherein the first die provides a MEMS component.
 27. The method of claim 24, wherein the first die is formed from a SOI structure, wherein the first device layer of the first die is formed from a silicon epitaxy layer of the SOI structure, the first dielectric layer of the first die is a buried oxide layer of the SOI structure, and the first silicon substrate of the first die is a silicon substrate of the SOI structure. 